As well known, car radios are frequently stolen, particularly from unattended parked vehicles. Various types of mechanical as well as electronic arrangements have been proposed which are intended to prevent unauthorized removal of the audio equipment - hereinafter for short "car radios"--or to so construct the car radios that, if removed, they then become useless and cannot be re-used. Thus, theft is intended to be prevent by so designing the radio that only an authorized user can operate it.
The referenced U.S. Pat. No. 4,720,700, assigned to the assignee of the present application and the disclosure of which is hereby incorporated by reference, describes a system and a method to inhibit operation of a radio upon unauthorized removal from a vehicle. The car radio includes a memory which has a code stored therein and which prevents operation of the apparatus if, after removal of the radio, the appropriate code word is not re-entered into the car radio. Removal of the car radio from a vehicle can be sensed either by permanent interruption of current supply from the vehicle battery or by sensing a certain position of the car radio in the vehicle, as described by coupling a potentiometer in the car radio, to the vehicle chassis so that, upon removal of the car radio, the setting of the potentiometer will be disturbed and, thus, the previously indicated code work which depended on the potentiometer setting, will be altered. The system works well; it has been found, however, in actual use that if the radio is removed by an authorized repair station, and is then re-inserted, it is difficult to reset the original potentiometer position. High mechanical precision of relatively movable parts is necessary which is expensive.
If the security of the car radio depends only on current supply, it has been found that some skilled thieves can override the current supply switches by supplying external battery power so that, upon artificially energizing the car radio before stealing it, the original code word will remain in the car radio and thus its operational utility will be preserved.